Recovery of carboxylic acid from organic solution that contains an amine and and an extraction enhancer

ABSTRACT

Recovery of carboxylic acid from an amine-based, water-immiscible organic extractant solution thereof that contains an extraction enhancer, by extraction of acid into an aqueous phase. In a preliminary step the enhancer is removed from the extractant solution by extraction with an aqueous solution containing at least 50 % by weight of a salt of the carboxylic acid present in the extractrant. The process is applicable to the recovery of carboxylic acid from a fermentation broth.

FIELD OF THE INVENTION

The present invention concerns quite generally the recovery of acarboxylic acid from an amine-based, water-immiscible organic extractantsolution thereof obtained by liquid-liquid contact extraction ofcarboxylic acid from an aqueous starting solution. The amine-basedextractant used for the extraction of the starting solution contains (i)a primary, secondary or tertiary long-chain alkyl amine in which theaggregate number of carbon atoms is at least 20; (ii) a liquidhydrocarbon; and (iii) a polar, non-carboxylic organic compound whichduring the extraction of the carboxylic acid from the aqueous startingsolution serves as extraction enhancer. One typical, but not exclusive,field of application of the invention is the recovery of a carboxylicacid from an aqueous fermentation broth, comprising in a first stage aliquid-liquid contact extraction of the fermentation broth with anextractant of the kind specified and in a second stage back-extractionof the carboxylic acid from the organic extractant solution into anaqueous phase.

BACKGROUND OF THE INVENTION

The extraction of carboxylic acid from aqueous solutions by amine-based,water-insoluble organic extractants was described for the first time bySmith and Page, J. Soc. Chem. Ind, 67.48 (1948). Since then, numerousstudies were published and a number of industrial processes established.Typical of the latter is the recovery of citric acid from fermentationbroths described in U.S. Pat. No. 4,275,234 (Baniel, et al.), accordingto which the acid is extracted at low temperature with anamine-containing, water-immiscible organic extractant and subsequentlyrecovered as an aqueous solution by back-extraction with water at ahigher temperature. As described in U.S. Pat. No. 4,275,234 and asindeed has become common practice, the extraction power of anamine-containing organic extractant is enhanced by the incorporation ofa non-carboxylic, neutral polar organic compound, e.g. an alkarol suchas n-butanol, a ketone such as butanone, an ester such as butylacetate,an ether such as dibutylether, a bifunctional compound such as CH₃ CH₂CH₂ CH₂ OCH₂ CH₂ OH etc. Such compounds, generally referred to asenhancers, modifiers or active diluents, increase the base strength ofthe amine in the extractant and thereby facilitate the transfer ofcarboxylic acid from the starting aqueous solution such as afermentation broth, into the organic extractant phase. Put in otherwords, the presence of an extraction enhancer shifts the carboxylic acidequilibrium in an aqueous phase/organic extractant phase system in favorof the organic phase. This very shifting of equilibrium, however,creates a problem for the back-extraction in that the transfer of thecarboxylic acid from the organic-to the aqueous phase is inhibited. Infact, this inhibition may be so pronounced as to render back-extractionof the organic acid with water impractical even at temperatures close to100° C.

Several approaches have been proposed to overcome this difficultyinherent in carboxylic acid recovery processes of this kind. Accordingto one extreme approach, back-extraction is foregone altogether andcarboxylic acid is recovered from the organic extract by distillation.Obviously, this procedure can be considered only for stable, relativelyvolatile acids such as acetic acid.

By another approach, back-extraction is carried out above the waterboiling temperature so as to increase the degree of hydrolysis of theamine-carboxyl complex and thereby provide for an acceptable rate ofback-extraction. This approach requires operation at above atmosphericpressure which is inconvenient and costly in terms of equipment andprocess control.

By yet another approach the enhancer is removed from the organic phaseby distillation prior to back-extraction as described in an extensivestudy "Extraction of Carboxylic Acids with Amine Extractants", Ind. Eng.Chem. Res. 1990, 29, 1319-1338 in the context of what is described thereas "diluent swing". This approach requires that the extractant be socomposed that the enhancer boils well below all other constituents andthat no decomposition of amine, carboxylic acid or enhancer takes placeat the distillation temperature. Even where these requirements can bemet, costly, energy-consuming vacuum distillation is as a rule required.

To sum up, the two-stage recovery of carboxylic acid from an aqueousstarting solution involving extraction of the starting solution with anorganic amine-based extractant and then back-extraction of thecarboxylic acid from the organic extractant into an aqueous phase, posesa technical dilemma with regard to the use of extraction enhancers:while on the one hand the use of an extraction enhancer is highlydesirable in the first stage for the purpose of increasing theextraction yield, it impedes the performance of the second stage. Therehas thus been a long-felt need to solve the problem and provide a methodby which carboxylic acid can be effectively recovered from an organic,water-immiscible amine-based extractant solution that also contains anextraction enhancer of the kind specified. It is the object of thepresent invention to provide such a method.

BRIEF DESCRIPTION OF THE INVENTION

The invention makes use of an effect by which salts in aqueous solutionmay act as solubilizer and which, broadly speaking, is a reversal of thesalting out effect. Salting out, which is a well known and widelyapplied effect, means that where a salt of good water solubility isdissolved in an aqueous solution holding a less soluble solute, thelatter will precipitate increasingly with the increase of the amount ofdissolved salt. In certain cases, where the concentration of the addedsalt exceeds a critical value at which the salting out is at its highestand the amount of solute remaining in solution at its lowest, a furtherincrease of the concentration of the added salt has been observed tocause solubilization of the solute with the result that a stablesolution forms from which the solute is not readily separable. Such asolubilization effect does occur only in some cases and it isunpredictable when it will occur and when not.

The salting out effect can be demonstrated by the precipitation of ann-propanol and n-butanol from their aqueous solutions upon the additionof NaCl, as demonstrated in the following Table 1 in which the data arecopied from R. DeSantis et al., The Chem. Eng. J., 1976 pp. 207-214:

                  TABLE 1                                                         ______________________________________                                        NaCl, wt %  0         10     15    20  25.5*                                  ______________________________________                                        Solubility of nPrOH                                                                       infinite  17     9     6   3                                      Solubility of nBuOH                                                                       8         3.2    2.2   1.4 0.8                                    ______________________________________                                         *Saturation value                                                        

It follows from the above Table 1 and from data in many otherpublications, that the solubility of alkanols and other neutral organiccompounds in water is consistently decreased with the increase of theamount of an added inorganic salt, all the way up to saturation of thatsalt.

With salts of carboxylic acids, however, the situation can be verydifferent. Thus, when NaCl is replaced by sodium lactate one notes asimilar salting-out effect up to about 40% salt concentration, but asthe lactate concentration increases further beyond that critical point,the solubilities of n-propanol and n-butanol invert and they actuallyincrease after going through minima, as shown in the following Table 2:

                  TABLE 2                                                         ______________________________________                                        Na-Lactate, wt %                                                                           0       10      20  30   40  50  60                              ______________________________________                                        Solubility of nPrOH                                                                        infinite                                                                              high    19  12   9   11  21                              Solubility of nBuOH                                                                        8       4       3   2    1.5 2   3                               ______________________________________                                    

The above effect is even more pronounced with K-lactate as shown in thefollowing Table 3:

                  TABLE 3                                                         ______________________________________                                        K-Lactate, wt %                                                                            0       10      20  30   40  50  60                              ______________________________________                                        Solubility of nPrOH                                                                        infinite                                                                              high    23  21   13  19  >40                             Solubility of nBuOH                                                                        8       4       3   2    2   2   5                               ______________________________________                                    

The difference between the solubilization effect of sodium and potassiumlactate illustrates a second important feature, namely that thesolubilization effect of carboxylic acid salts vary for any givencarboxylate with the nature of the cation.

In accordance with the present invention it has now surprisingly beenfound that a solubilization effect does occur in solutions of carboxylicacids in amine-based extractants which also contain an extractionenhancer. It has further been found that this effect can be adapted forthe selective extraction of the enhancer from the organic,water-immiscible amine-based extractant solution of carboxylic acid, byperforming the extraction with an aqueous solution of a salt of thecarboxylic acid present in the extractant.

In accordance with the invention there is provided in a process ofrecovering a carboxylic acid from an amine-based, water-immiscibleorganic extractant solution thereof that contains an extractionenhancer, by extraction of the acid into an aqueous phase, theimprovement by which the recovery of the carboxylic acid is preceded byextraction of the enhancer from the extractant solution with an aqueoussolution containing at least 50% by weight of a salt of the carboxylicacid present in the extractant solution.

In terms of saturation percentage, the aqueous extractant solutionshould contain an amount of the said carboxylic acid salt correspondingto at least 50% and preferably at least 80% of the saturation value.

The cation of the carboxylic acid salt dissolved in the aqueousextractant solution can be selected from a large variety of metal andammonium cations. The salts are best formed in situ by adding to theaqueous extraction solution equivalent amounts of the free carboxylicacid and a base such as, for example, NaOH, KOH, (CH₃)₄ NOH and thelike. As a rule, lower mono-, di-, tri- and tetraalkyi ammonium saltsand lower mono-, di- and trihydroxy- alkyl ammonium salts are preferredsince such carboxylic acid salts are better soluble and provide for ahigher water solubility of the extractant enhancer. Also, the organicradical such as, for example, the CH₃ group in tetramethylammonium, canbe expected to contribute to the solubilization of an enhancer with ahydrocarbon moiety such as, for example, butanol. The contribution ofalkyl radicals in an ammonium cation to solubilization does, as a rule,increase with the size of the alkyl radicals.

If desired, the ammonium cation of the carboxylic acid salts may alsobear hydroxyl groups such as, for example, in the triethanolammoniumcation. The hydroxyl groups in such cations contribute to thesolubilization of an enhancer that contains a polar moiety able tointeract with a hydroxyl group by way of a hydrogen bond, a typicalexample being a keto group CO in ketonic enhancers such as butanone CH₃CH₂ COCh₃.

It is to be noted that the most appropriate cation moiety of acarboxylic acid salt to be incorporated in the aqueous solution used forthe selective removal of the enhancer from the extractant carboxylicacid solution in accordance with the present invention, as well asprocess parameters such as temperature, flow rate, residence time etc.,can be established in each case by simple experimentation on the basisof the teachings of the present invention.

The invention further provides in a process for the recovery of acarboxylic acid from a fermentation broth which comprises extraction ofthe fermentation broth with a water-immiscible, amine-based organicextractant that includes an extraction enhancer to yield an extractantsolution of carboxylic acid, and recovery of carboxylic acid therefromby back-extraction into an aqueous phase, the improvement by whichrecovery of carboxylic acid from the extractant solution is preceded byextraction of the enhancer from the extractant solution with an aqueoussolution containing at least 50% by weight of a salt of the carboxylicacid present in the extractant solution; to yield an enhancer-depletedextractant solution of carboxylic acid and an aqueous enhancer solution;the aqueous enhancer solution is separated; and the enhancer is regainedfrom the separated aqueous solution thereof and recycled.

In accordance with one embodiment, the regaining of the extractionenhancer from its aqueous solution is effected by liquid-liquidextraction with recycled, enhancer-free, amine based extractant and theresulting enhancer bearing extractant is recycled.

In accordance with another embodiment, the aqueous enhancer solution isdiluted with water whereby extractant separates out as a distinct phaseand is recycled.

In accordance with yet another embodiment of the invention applicable tooperations in which the boiling point of the enhancer or its azeotropewith water is lower than that of water, the enhancer is regained fromthe aqueous solution thereof by distillation.

DESCRIPTION OF THE DRAWINGS

For better understanding, the invention will now be described, by way ofexample only, in the accompanying drawings in which:

FIG. 1 is a graphical representation illustrating the solubilization ofan extraction enhancer by means of a carboxylic acid salt;

FIG. 2 is a block diagram illustrating in general terms the concept ofenhancer separation;

FIG. 3 is a block diagram illustrating in a general fashion theinvention; and

FIGS. 4, 5 and 6 are block diagrams of three different embodiments ofthe invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 is a graphic representation which plots the solubility of anorganic extraction enhancer such as, for example a lower alkanol orketone, as a function of the concentration of an added carboxylic acidsalt. Point (a) on the graph indicates the solubility of the enhancer inpure water, point (b) the amount of dissolved carboxylic acid salt thatresults in minimum solubility of the enhancer and point (c) thesolubility of the enhancer in a saturated aqueous solution of thecarboxylic acid salt.

FIG. 2 illustrates in general terms the concept of enhancer separation.As shown, an amine-based extractant which includes an extractionenhancer and is loaded with carboxylic acid, RCOOH is charged into aunit 1 for enhancer separation in accordance with the teachings of thepresent invention. The extractant emerging from unit 1 is depleted ofenhancer but is still fully loaded with carboxylic acid. This enhancerdepleted extractant is charged into a unit 2 for back-extraction withwater. The resulting aqueous carboxylic acid solution is withdrawn asproduct while extractant depleted of both carboxylic acid and enhanceris charged into an extractant reconstitution unit 3 where it is mixedwith recycled enhancer arriving from the enhancer separation unit 1. Thereconstituted extractant is withdrawn from unit 3 and is recycled toextraction of an aqueous starting solution.

FIG. 3 illustrates generally the enhancer separation operation inaccordance with the invention. As shown, the amine-based extractantwhich includes an extraction enhancer and is loaded with carboxylicacid, RCOOH is charged into an enhancer extraction unit 1a where it iscontacted with an aqueous solution of a salt CtOOCR where Ct is acation, say a tetraalkyl ammonium salt, of the same carboxylic acidpresent in extractant unit 1a. Amine-based extractant still loaded withfree carboxylic acid and depleted of enhancer is withdrawn from theenhancer extraction unit 1a and charged into unit 2 for back-extraction,similar as in FIG. 2. An aqueous solution containing the said carboxylicacid salt and extracted enhancer is withdrawn from unit 1a and fed intounit 1b where the enhancer is recovered and the remaining aqueouscarboxylic salt solution is withdrawn and recycled to unit 1a. Therecovered enhancer is fed into the extraction reconstitution unit 3concurrently withdrawn from back-extraction unit 2 which is depleted ofboth carboxylic acid and enhancer, and the remainder of the operation isagain as in FIG. 2.

FIG. 4 illustrates one embodiment of the recovery of the enhancer in theperformance of the method according to the invention. As shown, theenhancer recovery unit 1b of FIG. 3 is sub-divided into two subunits1b-1 and 1b-2. The aqueous solution containing carboxylic acid saltCtOOCR where Ct is a cation and enhancer withdrawn from an enhancerextraction unit such as unit 1a in FIG. 3, is charged into the firstsubunit 1b-1 where it is diluted with water. In consequence, part of theenhancer separates from the aqueous solution with the formation of twoliquid phases. The enhancer phase is withdrawn and fed into theextractant reconstitution unit 3. The remaining aqueous enhancersolution which is partially depleted of enhancer, is withdrawn fromsubunit 1b-1 and charged into subunit 1b-2 where it is contacted withamine-based extractant depleted of both carboxylic acid and enhancer andwhich is withdrawn from a back-extraction unit such as unit 2 in FIGS. 2and 3. By this contact the enhancer is extracted from the aqueoussolution and the enhancer-loaded extractant is merged with pureextractant withdrawn from subunit 1b-1 and charged into the extractantreconstitution unit 3.

A diluted aqueous solution of carboxylic acid salt is withdrawn fromsubunit 1b-2 for reconcentration and recycling.

A further embodiment of enhancer recovery is illustrated in FIG. 5. Asshown, an aqueous solution containing carboxylic acid salt CtOOCR whereCt is a cation and enhancer withdrawn from an enhancer extraction unitsuch as unit 1a in FIG. 3, is charged into a dilution unit concurrentlywith water which leads to the separation of enhancer from the aqueoussolution with the formation of two liquid phases. The enhancer phase iswithdrawn and sent to extractant reconstitution. An aqueous solutionpartially depleted of enhancer is withdrawn from unit 5 and charged intoa concentration unit 6 from where a reconcentrated aqueous carboxylicacid salt solution on the one hand and water and enhancer vapors on theother hand are separately withdrawn. The vapors are charged into acondensation and phase separation unit 7 from where condensed enhanceris withdrawn and merged with the enhancer arriving from the dilutionunit 5, and condensed water is separately withdrawn and rejected.

FIG. 6 shows an overall process for the recovery of carboxylic acidRCOOH from an aqueous fermentation broth, embodying an embodiment of themethod according to the invention in which the enhancer is recovered bydistillation.

Briefly, the installation includes an extraction unit 9 for theextraction of an aqueous carboxylic acid feed with an amine-based,water-immiscible organic extractant, a unit 10 for the extraction of theenhancer from the acid-loaded extractant in accordance with theteachings of the invention, a unit 11 for the distillation of theenhancer, a back-extraction unit 12, an enhancer recovery unit 13 and anextractant reconstitution unit 14.

In operation crude aqueous carboxylic acid is subjected to liquid-liquidextraction in unit 9 with an amine-based, water-immiscible organicextractant that contains an extraction enhancer, and the resultingacid-loaded extract is fed into unit 10 for the extraction of theenhancer with an aqueous solution of the salt of the carboxylic acidcontained in the extractant in accordance with the teachings of thepresent invention. The enhancer-loaded aqueous solution is conductedinto unit 11 where the enhancer is distilled off and the remainingenhancer-depleted aqueous solution is returned to the extraction unit 10for extraction of the enhancer.

Extractant loaded with carboxylic acid and depleted of enhancer ischarged into the back-extraction unit 12 concurrently with water, andthe resulting aqueous carboxylic acid solution is withdrawn as product.

An aqueous raffinate which holds some dissolved enhancer is withdrawnfrom the extraction unit 9 and charged into the enhancer recovery unit13 together with amine-based extractant withdrawn from theback-extraction unit 12. In consequence of the contact between the twophases, the enhancer is re-extracted into the extractant and the sopartly reconstituted extractant is charged into reconstitution unit 14where it is mixed with further amounts of extractant withdrawn from thedistillation unit 11, and the so-reconstituted extractant is recycledinto the extraction unit 9.

An aqueous residue is withdrawn as waste from this enhancer recoveryunit 13.

The foregoing specific disclosure clearly teaches the salient featuresof the method of the present invention and on the basis thereof a personskilled in the art can readily select reactants and process parametersfor his specific needs with the aid of only some rudimentaryexperiments.

The present invention enables effective use of low molecular weight,highly water-soluble and volatile enhancers such as ethanol and acetone,which in certain cases may provide for a further advantage. Thus, forexample, ethanol at 3% concentration in a given amine-based extractantis approximately equivalent to 10% of octanol in the same extractant.However, if the carboxylic acid needs to be recovered from a diluteaqueous solution such as a fermentation broth, an enhancer such asethanol will report significantly to the aqueous extraction residueraffinate, thereby imposing an onerous enhance recovery operation bydistillation of the raffinate, in addition to the enhancer separationfrom the organic carboxylic acid extract. In the scheme shown in FIG. 6a volatile enhancer is partly recovered only from the concentratedaqueous solution of carboxylic acid salt solution withdrawn from unit 10by distillation, which is much more effective than recovery of the samevolatile enhancer from a large volume of extractant loaded withcarboxylic acid, and the rest of the enhancer that had separated to theraffinate is regained therefrom by extraction in the manner shown inFIG. 6.

I claim:
 1. In a process of recovering a carboxylic acid (1) from anamine-based, water-immiscible organic extractant solution (2) thereofthat contains an extraction enhancer, by extraction of the carboxylicacid (1) into an aqueous phase, the improvement whereinthe recovery ofthe carboxylic acid (1) is preceded by extraction of the extractionenhancer from the organic extractant solution (2) with an aqueoussolution (3) containing at least 50% by weight of a salt of saidcarboxylic acid (1) present in said extractant solution.
 2. The methodof claim 1, wherein the amount of carboxylic acid salt in the aqueoussolution corresponds to at least 50% of the saturation value.
 3. Themethod of claim 1, wherein the amount of carboxylic acid salt in theaqueous solution corresponds to at least 80% of the saturation value. 4.The method of claim 1, wherein said carboxylic acid salt is a member ofthe group of mono-, di-, and tetraalkyl ammonium salts.
 5. The method ofclaim 1, wherein said carboxylic acid salt is a trihydroxyalkyl ammoniumsalt.
 6. In a process for the recovery of a carboxylic acid (1) from afermentation broth which comprises extraction of the fermentation brothwith a water-immiscible, amine-based organic extractant that includes anextraction enhancer to yield an organic extractant solution (2) ofcarboxylic acid (1), and recovery of said carboxylic acid (1) therefromby back-extraction into an aqueous phase (3), leaving behind an aciddepleted fermentation broth, the improvement whereinrecovery of saidcarboxylic acid (1) from said organic extractant solution (2) ispreceded by extraction of the extraction enhancer from the organicextractant solution (2) with an aqueous solution containing at least 50%by weight of a salt of the carboxylic acid (1) present in the organicextractant solution (2) to yield an extraction enhancer-depleted organicextractant solution (2) of carboxylic acid (1) and a separate aqueousenhancer solution (3); and the enhancer is regained from the separateaqueous enhancer solution (3) and recycled leaving behind a residualaqueous phase.
 7. The process of claim 6, wherein regaining of theextraction enhancer from said separate aqueous solution is effected byliquid-liquid extraction with recycled enhancer-free, amine-basedextractant and the resulting enhancer bearing extractant is recycled. 8.The process of claim 6, wherein said separate aqueous enhancer solutionis diluted with water whereby extractant separates out as a distinctphase and is recycled.
 9. The process of claim 8, wherein said residualaqueous phase is subjected to extraction with recycled enhancer-freeextractant.
 10. The process of claim 6, wherein the boiling point of theenhancer is lower than that of water, and the enhancer is regained fromsaid separate aqueous solution thereof by distillation.
 11. The processof claim 10, wherein enhancer is recovered from said acid depletedfermentation broth by extraction with recycled extractant free of bothcarboxylic acid and enhancer.
 12. The method of claim 2, wherein saidcarboxylic acid salt is a mono-, di-, or tetraalkyl ammonium salt. 13.The method of claim 3, wherein said carboxylic acid salt is a mono-,di-, or tetraalkyl ammonium salts.
 14. The method of claim 2, whereinsaid carboxylic acid salt is a trihydroxyalkyl ammonium salt.
 15. Themethod of claim 3, wherein said carboxylic acid salt is atrihydroxyalkyl ammonium salt.